A high speed turbo machine, such as, for example, a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the machine. Due to their complex design, natural resonant mechanical frequencies of the blades may coincide with or be excited by certain blade rotational speeds and rotational harmonics thereof. To prevent excessive vibration of the blade about its normal position, prudent design practice dictates that the blades be constructed such that the frequencies of the lowest modes fall between harmonics of the operating frequency of the turbine. In addition, the blades may be excited by non-synchronous forces such as aerodynamic buffeting or flutter. In order to avoid the vibration exceeding certain levels and setting up objectionable stresses in the blades, it is common to monitor the vibrations of the blades, both during the design and testing of the turbine and during normal operation of the turbine. For example, it is known to use non-contacting proximity sensors or probes to detect blade vibrations. The probes detect the actual time-of-arrival of each blade as it passes each probe and provide corresponding signals to a blade vibration monitor system (BVM). Small deviations due to vibration are extracted, from which the BVM may determine the amplitude, frequency, and phase of the vibration of each blade.
The measured vibration amplitude is highly dependent on correct positioning of the sensor above the blade target, which may comprise a target affixed to the blade, a feature of the blade or the blade tip itself. Further, there is a current interest in obtaining measurement of high frequency vibration modes that result in failures of small gas turbine compressor blades, unlike lower vibration modes that result in movement of the blade as a whole, these high frequency modes form closely spaced nodes and antinodes across the compressor blades. The distance between nodes, i.e., between the BVM maximum measurements, and antinodes, i.e., between no BVM measurements, can be centimeters. Unlike lower modes, it is imperative that the actual point measured by the BVM on the blade be known in order to use this measurement as a boundary point to calculate the maximum stress in the blade and hence accurately determine loss of blade life.
In one known system of obtaining time-of-arrival data from rotating blades, a five lens laser line probe spreads a laser light into a line that spans a portion of the blade tip to be certain that either the leading blade tip edge or trailing blade tip edge is detected as the time-of-arrival. A pulse of light is produced by the laser light reflected from the tip edge as it passes the probe, and is received by the probe. If the probe is positioned over the leading edge, a leading pulse edge indicates the arrival of the leading blade tip edge, providing a vibration measurement at the leading tip edge. If the probe is positioned over the trailing blade tip edge, a falling pulse edge indicates the leaving trailing blade tip edge, providing a vibration measurement at the trailing tip edge. Such a measurement requires that the line of laser light overhang the leading or trailing edge to ensure that the leading or trailing edge is intercepted, in that a “missed edge” condition will not be detected by this probe. In addition, the measurements provided by these line probes are limited to the vibration occurring at the leading or trailing edge, and the reflected signals provided by these probes are weak since the laser light must be distributed over a line, rather than being provided to a single small point on the blade tip edge.
It is desirable to be able to obtain vibration measurements at various locations along the length of the blade tip, in addition to the blade tip edges, and it is also important to accurately know where the vibration is measured along the blade tip to enable calculation of vibrations elsewhere in the blade and to determine the maximum stress developed in the blade. Current vibration measurement techniques generally do not provide accurate information as to the exact position along the blade tip of the vibration measurements. Further, this unknown position of the vibration measurements changes as the rotor moves axially with respect to the case mounted sensor due to spins up and down, as the turbine warms up and cools down, and as the load on the turbine changes. Hence, it is not possible to accurately predict vibration elsewhere in the blade, or determine the maximum stress developed in the blade.